Skintones / Histogram

Started Mar 16, 2008 | Discussions thread
RDKirk Forum Pro • Posts: 14,834
Re: Skintones / Histogram

alfredky wrote:

How would the white towel method work if you have a scene which is
predominantly dark?

The purpose of the white towel method is simply to determine the maximum exposure that is just short of blowing out the brightest highlight that must retain detail. We know that because of the way digital sensors work, when we place that tone that is the "brightest highlight that must retain detail" at the top end of the sensor's range, we will have placed as much of the scene as possible in that important high end where the sensor can store the most data.

This is the same thing as "expose to the right," except that we've done it intelligently by choosing the proper tone and specifically placing it on the histogram.

However, "brightest highlight that must retain detail" is up to the artistic choice of the photographer. In most cases, it will be something white with detail--the white towel being a good representative most of the time. What is "expose to the right?"

But let's say I'm photographing a chocolate Labrador Retriever against a black background with soft lighting. I might NOT want to introduce a white towel as the "brightest highlight that must retain detail" because in that image it doesn't represent that actual "brightest highlight that must retain detail."

If I used a white towel for that scene, it would not pull the image up into the high end of the sensor's range--all the image detail would still be captured in the low end amid the noise. But because there aren't any brighter highlights I'm worried about, I can give it more exposure--get the image detail safely above the noise--and adjust the image easily in edit.

In all cases, I'm still basing exposure on selecting the "brightest highlight that must contain detail" in that particular scene and placing it at the top end of the histogram (usually that will be represented by a white, detailed surface...but maybe not--photographer's choice).

This is the fundamental difference between exposing for film and exposing a digital sensor, because you will find that the proper digital exposure is frequently OVEREXPOSURE for film.

That's why a gray card or an incident reading is not the best instrument for digital exposure--those readings are often underexposure by as much as a stop. That underexposure means the digital shadows fall deeper into the noise, resulting in diminished dynamic range.

To get the greatest dynamic range any digital sensor is capable of producing, you must move the data as high on the sensor's range as possible without overshooting the top. So it's the "brightest highlight that must contain detail" that is our anchor for digital exposure. Get that right, and you've got the best exposure you can get.

(HOWEVER, a well-lighted chocolate Lab on a black background should actually have some important true highlights, so in actual practice even this example isn't the best.)

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